As LSI circuits are increasing in density, the line width of circuits of semiconductor devices is becoming finer. Examples of methods of producing an exposure mask (also called a reticle that is used in a stepper or a scanner) to be used to form a circuit pattern for such a semiconductor device include an electron beam writing technique with high resolution.
As an electron beam writing apparatus, a writing apparatus using multiple beams has been developed as a replacement for an existing single beam writing apparatus that deflects one beam and irradiates a necessary location on a substrate with the beam. By using multiple beams, irradiation is possible with more beams than in the case where writing is performed using one electron beam, thus resulting in a significantly improved throughput. In a multi-beam writing apparatus, for example, an electron beam emitted from an electron gun is caused to pass through an aperture member having a plurality of holes to form multiple beams, blanking control is performed on each beam at a blanking aperture array, a beam that is not blocked is reduced using an optical system, and a substrate placed on a movable stage is irradiated with the beam.
In such a multi-beam writing apparatus, multiple beams are applied simultaneously to an aperture member, the beams pass through the same or different apertures of the aperture member to form beams, and the formed beams are connected, thus forming a figure pattern having a desired shape. The shape of an entire image of multiple beams (hereinafter, also referred to as “beam shape”) applied to a substrate represents the connection accuracy of a written figure. It is therefore important to compensate for distortion of the entire image of multiple beams.
To compensate for distortion of the entire image of multiple beams, the beam shape has to be measured accurately. Typically, the beam shape is measured by sequentially switching beams to a beam ON state, scanning each beam over a reflection mark on a stage to detect reflected electrons, and calculating the positions of the beams.
In scanning over the reflection mark on the stage, however, the amount of beam deflection by a deflector may increase and the trajectory of a beam may change accordingly, causing distortion of the beam shape. This distortion leads to a reduction in accuracy of beam position measurement.
The number of multiple beams is large (for example, approximately, 260,000 beams). It is therefore extremely difficult to fabricate an aperture member for forming multiple beams and a blanking aperture array member for performing blanking control on beams such that the members have no defects. In some cases, a defect of the aperture member or the blanking aperture array member causes an unintended beam to reach a substrate or causes a necessary beam to be interrupted, or prevented from reaching the substrate. It is difficult to eliminate effects of such a defect on the accuracy of beam position measurement in the typical method including scanning over a reflection mark. There is a need for a new method of accurately measuring the positions of beams.